colibri_vf: fix sdboot for vybrid modules

Currently, Vybrid's sdboot variable tries to load the kernel from /boot
of the root partition (typically second partition when using the sdcard
image). However, since we moved to flash the kernel in a separate UBI
volume, we no longer deploy the kernel/device tree to /boot, hence
sdboot does not work in its current state.

Load the kernel and device tree from the first (typically FAT) partition
as customary on all Toradex modules.

While at it also change from rw to ro as e.g. systemd will re-mount the
root file system rw anyway after checking it.

Signed-off-by: Gerard Salvatella <gerard.salvatella@toradex.com>
Acked-by: Stefan Agner <stefan.agner@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@toradex.com>